COORDINATION 



433 



Electric organs 



Powerful electric discharges can be pro- 

 duced by a few species of fish. They are 

 "triggered" by stimuH coming from the 

 nervous system. The electric eel and ray 

 possess special "electric organs" that are 

 composed of a great many modified muscle 

 cells so arranged as to accumulate their in- 

 dividual action currents and build up a 

 considerable voltage. In some forms as 

 much as 400 volts have been recorded, 

 which is sufficient to stun or even kill a 

 small fish. When a small light bulb is placed 

 in the circuit, flashes have been recorded. 

 Horses wading through shallow water 

 where the large electric eel resides have 

 been shocked enough to throw their riders. 



Muscles and glands occur in all species 

 of the Metazoa. The additional effectors — 

 pigmentary, luminescent, and electric — are 

 much more restricted in their occurrence. 

 They demonstrate how far animals may be 

 able to modify certain parts of their bodies 

 to perform very special functions. It is in- 

 teresting to speculate how, through the mil- 

 lions of years of evolution, these animals 

 have been able to select such aberrant, 

 though practical, modifications. 



CHEMICAL COORDINATION- 

 THE ENDOCRINE GLANDS 



An important adjunct to the nervous sys- 

 tem in bringins; about coordination of the 

 vastly complex animal body is the endo- 

 crine system. This is made up of glands 

 located in various regions of the body 

 which secrete powerful organic compounds 

 directly into the blood stream ( Fig. 16-29 ) . 

 Their activity is manifest in other parts of 

 die body. While the nervous system is re- 

 sponsible for quick action, the endocrine 

 system functions in bringing about the 

 much slower reactions which may extend 

 over some period of time. 



The glands which are known to be endo- 

 crine in nature today were described by 



early anatomists. Thus, Galen, in the second 

 century a.d., described the tiny pituitary 

 gland of mammals, although he could 

 assign no function to it. Indeed, it was not 

 until nearly the end of the last century 

 that actual experimentation began to bring 

 to light the function of these mysterious 

 glands. 



The endocrine glands and their secretions 



The endocrines evolved after the nerv- 

 ous system, so it might be expected that 

 they would be found in the more highly 

 specialized animals where the nervous sys- 

 tem could not take care of the multitudi- 

 nous jobs assigned to it. These glands are 

 present in Crustacea and insects (Chapter 

 11) and they may be important for coordi- 

 nation or other functions in animals lower 

 than the arthropods, but as yet their pres- 

 ence has not been demonstrated. They are 

 consistently found among the vertebrates, 

 even in such low forms as the cyclostomes. 

 The glands themselves are derived em- 

 bryologically from various sources, and in 

 their evolution have performed different 

 functions. For example, the hormones that 

 control chromatophore activity in the 

 amphibia can exercise no comparable func- 

 tion in birds and mammals because they 

 have no chromatophores. Yet, the hormone 

 is still present, as can readily be demon- 

 strated by the injection into a frog of the 

 proper extract. Undoubtedly, such re- 

 cently acquired hormones as those that 

 stimulate lactation in mammals are de- 

 rived from similar hormones present in 

 lower vertebrates where they perform a dif- 

 ferent function. There has been a long, 

 slow, biochemical evolution of these com- 

 plex substances which have an intricate 

 interrelationship in the higher animals 

 today. This very complex relationship has 

 been the subject of a tremendous amount of 

 research during the past 50 years. 



The vertebrates have seven clearly recog- 

 nized endocrine glands: the gonads, pan- 



